It is forbidden to use referral links in the post body, you may only place them in your signature.
Correct, it is mentioned in the 4th point in the rules:
Unofficial list of (official) Bitcointalk.org rules, guidelines, FAQ4. No referral code (ref link) spam. [1]
An alternative that OP can do is to use the affiliate link in the signature space. He can have someone make him a signature code and add a referral link to the casino he is promoting, that is much better and advisable. Referral links posted in the body of the post will be removed for violating the rules.